A number of techniques have been developed by which a material may be deposited on a substrate. One of these techniques is referred to generally as chemical vapor deposition. In chemical vapor deposition, a gaseous species in a chamber is caused to precipitate or deposit on a heated substrate or target in the chamber under precisely controlled conditions. In one type of chemical vapor deposition process, the gaseous species which forms a deposit on the substrate is a gas plasma. As those skilled in the art will appreciate, a gas plasma is made up of highly reactive species which include free electrons, ions, and radicals. Thus, a gas plasma or plasma is in essence a mixture of electrically neutral and charged particles.
One of the major applications of chemical vapor deposition processes is the fabrication of semiconductor devices for the electronics industry. Other applications include the fabrication of gas sensors, optical devices, accelerometers and thin functional and decorative coatings, all of which may also be manufactured using various thin-film techniques.
In addition to the many traditional materials processed by chemical vapor deposition, more recently it has been found that synthetic diamond can be formed in this manner. The formation of synthetic diamond is of considerable significance due to its desirable thermal and electrical properties, the latter being controllable through the introduction of dopant impurities. This interest in synthetic diamond has resulted in the development of several advances in chemical vapor deposition technology, advances which are also useful in the fabrication of non-diamond films and articles.
With respect to the fabrication of synthetic diamond by chemical vapor deposition, it has been found that gas plasma deposition, and particularly microwave gas plasma deposition, provides the greatest control over critical parameters of the process. In this regard, a gas feedstock comprising hydrogen and a carbon-containing gas such as methane or acetylene are introduced into a vacuum chamber in which the deposition of synthetic diamond on a substrate is to be carried out. The deposition apparatus includes a microwave generator which emits electromagnetic radiation at a preselected energy level into the chamber. Microwaves excite the gaseous feedstock to produce a gas plasma. The feedstock gas dissociates to form hydrogen ions, free electrons and CH.sub.3 radicals, the latter serving as one of the precursors of the synthetic diamond deposit.
The substrate upon which a synthetic diamond deposit is to be formed is positioned within the vacuum chamber. In order for synthetic diamond to deposit on the substrate, the substrate must be heated to a predetermined temperature. Heat is supplied to the substrate, primarily by thermal conduction from a susceptor in the chamber which supports the substrate. The susceptor comprises a body of material which can be heated rapidly to extremely high temperatures by induction heating. Thus, the material from which the susceptor is formed must respond efficiently to inductive heating and exhibit thermal stability at high operating temperatures.
In the past, many susceptors for use in chemical vapor deposition processes have been formed of graphite. For example, one such prior art susceptor is in the form of a hollow cylinder having one open end. An induction coil is positioned in the susceptor cavity and serves to raise the susceptor to the necessary operating temperature by induction heating. The substrate, for example a silicon wafer, is placed on the closed end of the cylinder and is thereby heated primarily through conduction from the susceptor to the substrate.
The configuration of the deposition apparatus, including the placement of the substrate, is such that a gas plasma forms just above the principal surface of the substrate. Once the proper conditions are achieved for synthetic diamond deposition, diamond begins to deposit on the surface of the wafer. However, as the present inventor and others have recognized, it is difficult to achieve the uniform deposition of a material, particularly synthetic diamond, on a substrate using conventional susceptors. As will be appreciated by those skilled in the art, many properties of films are functions of film thickness. Particularly in the fabrication of microelectronic devices, it is essential that thin films of relatively uniform thicknesses be formed in a reliable and reproducible manner. Excessive variation in film thickness produces aberrant electrical characteristics which cannot be tolerated in microelectronic devices and unwanted distortion in optical devices. These variations in deposited films are a significant problem for the industry.
The present invention provides a unique solution to the problem of materials deposition uniformity which allows materials such as synthetic diamond to be deposited with far greater uniformity of thickness than was previously possible. This is achieved in the present invention by the use of a novel susceptor that promotes the formation of uniform films.